The documents U.S. Pat. No. 4,994,671, US2014/0361155 and the Technical Paper “Advanced Downhole Fluid Analysis” IBP3075_10 of Rio Oil & Gas Expo and Conference 2010 held 13-16 Sep. 2010 in Rio de Janeiro presented by Brazilian Petroleum, Gas and Biofuels Institute—IBP, Jesus A. Canas et al. describe Downhole Fluid Analysis (DFA) using a wireline formation tester tool, in particular a modular formation dynamics tester tool. A formation tester tool is used to take samples of reservoir fluids directly from hydrocarbon bearing zone. A formation tester tool that includes a DFA tool is used to measure physical and chemical properties of reservoir fluids during the sampling phase of the formation tester tool. The DFA tool enables having access to real time information on fluids during the sampling phase.
FIG. 1 is a cross-section view schematically illustrating a formation tester tool 1 including a DFA tool 2 sampling a reservoir fluid 3 flowing from a hydrocarbon bearing zone 4 into a hydrocarbon well 5 that has been drilled into an earth formation. Generally, the formation tester tools and DFA tools have a cylindrical shape that is adapted to travel into well bore hole. The formation tester tool 1 is suspended in the borehole of the well from a lower end of a logging cable or wireline spooled on a winch at the surface (not shown). The logging cable is coupled to a surface electrical control system having appropriate electronics and processing systems (not shown). The fluid 3 enters the formation tester tool 1 at a formation pad 6 applied to the wall of the well 5 in a sealed manner. The fluid 3 is pumped by means of a pump 7 though a downhole sampling flow line 8 extending internally of the formation tester tool 1. The fluid 3 is directed towards the downhole fluid analysis tool 2 for real time analysis and also towards a tool module storing samples in bottles for delayed analysis in surface laboratories. The remaining fluid 3 can also be discarded out of the formation tester tool 1 by an outlet 10 into the well 5.
The formation tester and the DFA tools operate in harsh environment, namely extreme conditions including high pressure from several hundred bars up to 2000 bars, high temperature up to 200° C., presence of corrosive fluids such as sulfuric acid, presence and contamination by solid particles such as scales, asphalthenes, sand particles, as well as multiphasic flow conditions (oil, gas, water). Further, there are also the space and power constraints associated to downhole tools deployment. Furthermore, there is the high shocks environment associated to wireline or drilling operations.
FIG. 2 is a detailed cross-section view schematically illustrating a DFA cell of the DFA tool of FIG. 1. The evaluation of fluid properties by the DFA tool 2 is based on the transmission of light 11 through the fluid sample and the measurement of the attenuation at different wavelengths. Optical absorption spectra are obtained that can be related to critical fluid characteristics such as for example Gas Oil Ratio, CO2 and other relative concentrations of chemical compounds. A state of the art DFA cell 12 for optical transmission measurements comprises sapphire windows 13, 14 assembled to metal body parts 15 made of high strength corrosion resistant alloys such as Inconel. The metal body parts 15 forms a fluid flow restriction. The emission module 16 comprises a light source 17 and filters 18. The reception module 19 comprises an optical fiber bundles collecting light to filters 21 and photo detectors 22 of a spectrometer. More recently continuous absorption spectra in the near infrared region have been developed based on gratings spectrometers specially engineered in order to withstand harsh conditions.
The drawbacks of such conventional sapphire windows approach are the following:                Low performance of compositional analysis—The optical path length selection (light travel distance inside the fluid) is constrained by the fluid flow restriction that can be accepted without altering the sampling process and/or the representativeness of the fluid sample present in between the two windows. Distance between windows below 1 mm is in practice impossible to implement and typical optical path lengths for current commercial tools are within the 3-5 mm range. This limitation has forced engineers to favor analysis in the visible-near infrared spectrum where attenuation factors in hydrocarbon fluids are relatively low, despite the fact that absorption peaks are related to overtones of the vibration modes of the molecules to be detected and suffer from peak enlargements and overlaps from different compounds. This leads to complex interpretation and relatively poor robustness and performance of compositional analysis.        Optical cell contamination—Relatively large optical window surface (several mm2) and even more importantly its large curvature ratio (superior to 1 mm) favor the formation of droplets of fluids sticking on the window surface and altering analysis.        Complex and expensive hardware—The window assembly (the conventional method is sapphire disk or tube brazing onto metal) is critical in order to withstand high pressure and high temperature.        Optical path—The optical path length is fixed and can only be changed at the manufacturing step of the cell.        Cleaning—The complex mechanical configuration of the cell flow line and windows leads to dead volumes which make the cleaning of the cell flow line during the early phase of the sampling a critical step.        